Accumulators located near a blow-out preventer (BOP) and other subsea equipment may be configured to provide pressure for operating hydraulic systems, such as the blow-out preventer (BOP). Subsea accumulators may store a combination of an inert gas and fluid. Initially, the subsea accumulator is charged with an initial pressure of gas, such as nitrogen. Fluid may then be pumped into the subsea accumulators to a final pressure, which may be equal to the BOP control system pressure. Compression of the gas within the subsea accumulator stores energy. The stored energy in the accumulator may be used to operate subsea equipment, such as when an emergency situation occurs resulting in a disconnect of energy from the surface. When the pressure of hydraulic fluid in the subsea system drops through use of the emergency system, the compressed gas expands, forcing the hydraulic fluid out of the accumulator and into the subsea system hydraulic lines.
When energy is supplied from the accumulators, in the absence of external energy such as from the surface, the pressure in the accumulators decreases over time as stored fluid energy is used for functions within the system. That is, as liquid is used from the accumulators, the pressure of the trapped gas decreases as a result of increasing volume for the gas, and the pressure within the subsea system hydraulic lines decreases. The decreased pressure in the fixed volume subsea system may result in limitations of components within the subsea system or through pressure limitations in the components or equipment used to convey the hydraulic fluid from the surface to the BOP. For example, a shear ram of a BOP may require a certain pressure level to shear a certain drillpipe in the event of an emergency. When that pressure level is not available from the accumulators, the BOP may fail to shear the drillpipe.
Additionally, when energy is supplied from the surface, the pressure within the subsea system may nevertheless be below an operating pressure for the subsea system. The drop in pressure from the surface to the subsea system may be due to leaks and other inefficiencies in the hydraulic fluid transfer system. Also, the drop in pressure may be from pressure limitations in the lines that convey the fluid from surface.
One conventional solution may be to increase the number of accumulators. Each additional accumulator provides an increase in the available volume of hydraulic fluid for operating the subsea systems. However, the additional accumulators may lead to an increased blowout preventer (BOP) stack weight and size, which is prohibitive to construction, installation, operation, and maintenance of the BOP or prohibitive to retrofitting additional accumulators onto a BOP stack. Thus, there is a need for providing increased pressure in a subsea system.